A method for producing l-arginine using a bacterium of enterobacteriaceae family, having attenuated expression of a gene encoding an l-arginine transporter

Abstract

 The present invention provides a method for producing L-arginine using a bacterium of the Enterobacteriaceae family, particularly a bacterium belonging to the genus Escherichia or Pantoea, which has been modified to attenuate expression of one or several genes encoding an L-arginine transporter.

Description

Background of the Invention

Field of the Invention

[0001] The present invention relates to the microbiological industry, and specifically to a method for producing L-arginine. The method uses a bacterium of the Enterobacteriaceae family which has been modified to attenuate expression of one or several genes encoding an L-arginine transporter.

Description of the Related Art

[0002] Conventionally, L-amino acids are industrially produced by fermentation methods utilizing strains of microorganisms from natural sources, or mutants thereof. Typically, the microorganisms are modified to enhance production yields of target L-amino acids.

[0003] Many techniques to enhance L-amino acid production yields have been reported, including transformation of microorganisms with recombinant DNA (see, for example, US patent No. 4,278,765). Other techniques for enhancing production yields include increasing the activities of enzymes involved in amino acid biosynthesis and/or desensitizing the target enzymes of the feedback inhibition by the produced L-amino acid (see, for example, WO 95/16042 or US patent Nos. 4,346,170; 5,661,012 and 6,040,160).

[0004] Other ways to enhance L-amino acid production yields is to attenuate expression of a gene or several genes which are involved in degradation of the target L-amino acid, genes which divert the precursors of the target L-amino acid from the L-amino acid biosynthetic pathway, genes involved in the redistribution of the carbon, nitrogen, and phosphate fluxes, and genes coding for toxins, etc.

[0005] A transport system dependent on a binding protein in Escherichia coli specific for L-arginine was characterized by genetic and biochemical means. The system is made up of five adjacent genes, artPIQMJ ("art" stands for arginine transport), which are organized in two transcriptional units (artPIQM and artJ). The artI and artJ gene products, ArtI and ArtJ, are periplasmic binding proteins with sequence similarity to binding proteins for polar, or basic amino acids. The artQ, artM, and artP products are similar to known transmembrane proteins and the ATPase of binding-protein-dependent carriers. The mature ArtI and ArtJ proteins are localized in the periplasm and lack a signal peptide of 19 amino acid residues. ArtI and ArtJ were isolated from overproducing strains. ArtJ specifically binds L-arginine with high affinity and overproduction of ArtJ stimulated L-arginine uptake by the bacteria. The substrate for ArtI is not known, and isolated ArtI did not bind common amino acids, various basic uncommon amino acids, or amines. It was concluded that the artPIQM artJ genes encode a third arginine-uptake system in addition to the known argT hisJQMP system of Salmonella typhimurium and E. coli and the arginine (-ornithine) carrier (aps) of E. coli (Wissenbach U. et al., Mol Microbiol.;17(4):675-86(1995)).

[0006] But currently, there have been no reports of attenuating expression of a gene encoding an L-arginine transporter for production of L-arginine.

SUMMARY OF THE INVENTION

[0007] An aspect of the present invention includes enhancing the productivity of L-arginine producing strains and providing a method for producing L-arginine acid using these strains.

[0008] The present invention provides a bacterium of the Enterobacteriaceae family having an increased ability to produce L-arginine.

[0009] It is an aspect of the present invention to provide an L- arginine producing bacterium of the Enterobacteriaceae family, wherein the bacterium has been modified to attenuate expression of one or several genes encoding an L-arginine transporter.

[0010] It is a further aspect of the present invention to provide the bacterium as described above, wherein said bacterium has been modified to attenuate expression of artI gene.

[0011] It is a further aspect of the present invention to provide the bacterium as described above, wherein expression of the artI gene is attenuated by inactivation of the artI gene.

[0012] It is a further aspect of the present invention to provide the bacterium as described above, wherein said bacterium has been modified to attenuate expression of artPIQM-artJ cluster.

[0013] It is a further aspect of the present invention to provide the bacterium as described above, wherein expression of the artPIQM-artJ cluster is attenuated by inactivation of the artPIQM-artJ cluster

[0014] It is a further aspect of the present invention to provide the bacterium as described above, wherein the bacterium belongs to the genus Escherichia.

[0015] It is a further aspect of the present invention to provide the bacterium as described above, wherein the bacterium belongs to the genus Pantoea.

[0016] It is a further aspect of the present invention to provide a method for producing L-arginine comprising:

• cultivating the bacterium as described above, and
• collecting L-arginine from the medium.

[0017] The present invention is described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] 

Figure 1 shows the relative positions of pairs of primers P1 and P2, and P5 and P6 on plasmid pMW118-attL-Cm-attR.

Figure 2 shows the construction of the chromosomal DNA fragment comprising the inactivated artI gene or artPIQM-artJ cluster.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

1. Bacterium of the present invention

[0019] The bacterium is an L-arginine producing bacterium of the Enterobacteriaceae family, wherein the bacterium has been modified to attenuate expression of one or several genes encoding an L-arginine transporter.

[0020] The term "L-arginine producing bacterium" means a bacterium which is able to produce and secrete L-arginine into a medium, when the bacterium is cultured in the medium.

[0021] The term "L-arginine-producing bacterium" also may mean a bacterium which is able to produce and cause accumulation of L-arginine in a culture medium in an amount larger than a wild-type or parental strain of a bacterium of the Enterobacteriaceae family, for example, E. coli, such as E. coli K-12, and preferably means that the bacterium is able to cause accumulation in a medium of an amount not less than 0.5 g/L, more preferably not less than 1.0 g/L of L-arginine.

[0022] The Enterobacteriaceae family includes bacteria belonging to the genera Escherichia, Enterobacter, Erwinia, Klebsiella, Pantoea, Photorhabdus, Providencia, Salmonella, Serratia, Shigella , Morganella and Yersinia, etc.. Specifically, those classified as Enterobacteriaceae according to the taxonomy used in the NCBI (National Center for Biotechnology Information) database (http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=91347) can be used. A bacterium belonging to the genus Escherichia or Pantoea is preferred.

[0023] The phrase "a bacterium belonging to the genus Escherichia" means that the bacterium is classified in the genus Escherichia according to the classification known to a person skilled in the art of microbiology. An example of a bacterium belonging to the genus Escherichia is, but is not limited to, Escherichia coli (E. coli).

[0024] The bacterium belonging to the genus Escherichia is not particularly limited, however for example, bacteria described by Neidhardt, F.C. et al. (Escherichia coli and Salmonella typhimurium, American Society for Microbiology, Washington D.C., 1208, Table 1) are encompassed.

[0025] The phrase "a bacterium belonging to the genus Pantoea" means that the bacterium is classified as the genus Pantoea according to the classification known to a person skilled in the art of microbiology. Some species of Enterobacter agglomerans have been recently re-classified into Parctoea agglomerans, Pantoea ananatis, Pantoea stewartii or the like, based on nucleotide sequence analysis of 16S rRNA, etc..

[0026] The phrase "bacterium has been modified to attenuate expression of one or several genes encoding an L-arginine transporter" can mean that the bacterium has been modified in such a way that the modified bacterium contains a reduced amount of the L-arginine transporter or any subunit thereof as compared with an unmodified bacterium, or it can also mean that the bacterium is unable to synthesize the L-arginine transporter or any subunit thereof.

[0027] The L-arginine transporter system is made up of products of five adjacent genes, artPIQMJ, which are organized in two transcriptional units, artPIQM and artJ. The artI and artJ gene products, ArtI and ArtJ, are periplasmic binding proteins with sequence similarity to binding proteins for polar, or basic amino acids. The artQ, artM, and artP products are similar to known transmembrane proteins and the ATPase of binding-protein-dependent carriers.

[0028] The phrase "inactivation of a gene" means that the modified gene encodes a completely non-functional protein. It is also possible that the modified DNA region is unable to naturally express the gene due to the deletion of a part of the gene or the gene entirely, a shifting of the reading frame of the gene, the introduction of missense/nonsense mutation(s), or the modification of an adjacent region of the gene, including the sequences controlling gene expression, such as the promoter, enhancer, attenuator, ribosome-binding site, etc..

[0029] The presence or absence of a gene on the chromosome of a bacterium can be detected by well-known methods, including PCR, Southern blotting, and the like. In addition, the level of gene expression can be estimated by measuring the amount of mRNA transcribed from the gene using various well-known methods, including Northern blotting, quantitative RT-PCR, and the like. The amount of the protein encoded by the gene can be measured by well-known methods, including SDS-PAGE followed by an immunoblotting assay (Western blotting analysis), and the like.

[0030] The artP gene (synonyms: ECK0855, b0864) encodes the ArtP protein subunit of the arginine ABC transporter (synonym B0864) located in the cytoplasm. The artP gene (nucleotides complementary to nucleotides 902,229 to 902,957 in the GenBank accession number NC_000913.2; gi:49175990; SEQ ID NO: 1) is located between the ybjP and artI genes on the chromosome of E. coli K-12 strain. The nucleotide sequence of the artP gene, and the amino acid sequence of ArtP protein encoded by the artP gene, are shown in SEQ ID NO: 1 and SEQ ID NO: 2, respectively.

[0031] The artI gene (synonyms: ECK0854, b0863) encodes the ArtI protein subunit of the arginine ABC transporter (synonym B0863) located in the periplasmic space. The artI gene (nucleotides complementary to nucleotides 901,480 to 902,211 in the GenBank accession number NC_000913.2; gi:49175990; SEQ ID NO: 3) is located between the artQ and artP genes on the chromosome of E. coli K-12 strain. The nucleotide sequence of the artI gene, and the amino acid sequence of ArtI protein encoded by the artI gene, are shown in SEQ ID NO: 3 and SEQ ID NO: 4, respectively.

[0032] The artQ gene (synonyms: ECK0853, b0862) encodes the ArtQ protein subunit of the arginine ABC transporter (synonym B0862) located in the inner membrane. The artQ gene (nucleotides complementary to nucleotides 900,757 to 901,473 in the GenBank accession number NC_000913.2; gi:49175990; SEQ ID NO: 5) is located between the artI and artM genes on the chromosome of E. coli K-12 strain. The nucleotide sequence of the artQ gene, and the amino acid sequence of ArtQ protein encoded by the artI gene, are shown in SEQ ID NO: 5 and SEQ ID NO: 6, respectively.

[0033] The artM gene (synonyms: ECK0852, b0861) encodes the ArtM protein subunit of the arginine ABC transporter (synonym B0861) located in the inner membrane. The artI gene (nucleotides complementary to nucleotides 900,089 to 900,757 in the GenBank accession number NC_000913.2; gi:49175990; SEQ ID NO: 7) is located between the artQ and artJ genes on the chromosome of E. coli K-12 strain. The nucleotide sequence of the artM gene, and the amino acid sequence of ArtM protein encoded by the artM gene, are shown in SEQ ID NO: 7 and SEQ ID NO: 8, respectively.

[0034] The artJ gene (synonyms: ECK0851, b0860) encodes the ArtJ protein subunit of the arginine ABC transporter (synonym B0860) located in the periplasmic space. The artJ gene (nucleotides complementary to nucleotides 899,067 to 899,798 in the GenBank accession number NC_000913.2; gi:49175990; SEQ ID NO: 9) is located between the artM and rlmC genes on the chromosome of E. coli K-12 strain. The nucleotide sequence of the artJ gene, and the amino acid sequence of ArtJ protein encoded by the artJ gene, are shown in SEQ ID NO: 9 and SEQ ID NO: 10, respectively.

[0035] Since there may be some differences in DNA sequences between the genera or strains of the Enterobacteriaceae family, the gene to be inactivated on the chromosome is not limited to the genes shown in SEQ ID NO:1, 3, 5, 7 or 9, but may include genes homologous to SEQ ID No:1, 3, 5, 7 and 9, which encode a variant protein of corresponding protein. The phrase "variant protein" means a protein which has changes in the sequence, whether they are deletions, insertions, additions, or substitutions of amino acids, but still maintains the activity of the product as the protein.

[0036] The number of changes in the variant protein depends on the position in the three dimensional structure of the protein or the type of amino acid residues. It may be 1 to 30, preferably 1 to 15, and more preferably 1 to 5. These changes in the variants are conservative mutations that preserve the function of the protein. In other words, these changes in the variants can occur in regions of the protein which are not critical for the function of the protein. This is because some amino acids have high homology to one another so the three dimensional structure or activity is not affected by such a change. A conservative mutation is a mutation wherein substitution takes place mutually among Phe, Trp, Tyr, if the substitution site is an aromatic amino acid; among Leu, Ile, Val, if the substitution site is a hydrophobic amino acid; between Gln, Asn, if it is a polar amino acid; among Lys, Arg, His, if it is a basic amino acid; between Asp, Glu, if it is an acidic amino acid; and between Ser, Thr, if it is an amino acid having a hydroxyl group. Typical conservative mutations are conservative substitutions. Examples of conservative substitutions include substitution of Ser or Thr for Ala, substitution of Gln, His or Lys for Arg, substitution of Glu, Gln, Lys, His or Asp for Asn, substitution of Asn, Glu or Gln for Asp, substitution of Ser or Ala for Cys, substitution of Asn, Glu, Lys, His, Asp or Arg for Gln, substitution of Asn, Gln, Lys or Asp for Glu, substitution of Pro for Gly, substitution of Asn, Lys, Gln, Arg or Tyr for His, substitution of Leu, Met, Val or Phe for Ile, substitution of Ile, Met, Val or Phe for Leu, substitution of Asn, Glu, Gln, His or Arg for Lys, substitution of Ile, Leu, Val or Phe for Met, substitution of Trp, Tyr, Met, Ile or Leu for Phe, substitution of Thr or Ala for Ser, substitution of Ser or Ala for Thr, substitution of Phe or Tyr for Trp, substitution of His, Phe or Trp for Tyr, and substitution of Met, Ile or Leu for Val. Substitutions, deletions, insertions, additions, or inversions and the like of the amino acids described above include naturally occurred mutations (mutant or variant) depending on differences in species, or individual differences of microorganisms that retain the artI gene. Such a gene can be obtained by modifying the nucleotide sequence shown in SEQ ID NO: 1 using, for example, site-directed mutagenesis, so that the site-specific amino acid residue in the protein encoded includes substitutions, deletions, insertions, or additions. The protein variant encoded by the gene may have a homology of not less than 80%, preferably not less than 90%, and most preferably not less than 95%, with respect to the entire amino acid sequence shown in SEQ ID NO. 2, 4, 6, 8 or 10 as long as the protein prior to inactivation is able to function as an arginine ABC transporter when complexed with the rest of 4 subunits of 5 wild-type proteins which compose the L-arginine transporter.

[0037] Homology between two amino acid sequences can be determined using well-known methods, for example, the computer program BLAST 2.0, which calculates three parameters: score, identity, and similarity.

[0038] Moreover, any of artP, artI, artQ, artM or art J genes may be a variant which hybridizes with the nucleotide sequence complementary to the sequence shown in SEQ ID NO: 1, 3, 5, 7 or 9, respectively, or a probe which can be prepared from the nucleotide sequence, under stringent conditions, provided that it encodes a functional protein prior to inactivation. "Stringent conditions" include those under which a specific hybrid, for example, a hybrid having homology of not less than 60%, preferably not less than 70%, more preferably not less than 80%, still more preferably not less than 90%, and most preferably not less than 95%, is formed and a non-specific hybrid, for example, a hybrid having homology lower than the above, is not formed. For example, stringent conditions are exemplified by washing one time, preferably two or three times at a salt concentration corresponding to 1x SSC, 0.1% SDS, preferably 0.1x SSC, 0.1% SDS at 60°C. Duration of washing depends on the type of membrane used for blotting and, as a rule, may be what is recommended by the manufacturer. For example, the recommended duration of washing for the Hybond^™ N+ nylon membrane (Amersham) under stringent conditions is 15 minutes. Preferably, washing may be performed 2 to 3 times. The length of the probe may be suitably selected depending on the hybridization conditions, and is usually 100 bp to 1 kbp.

[0039] Expression of a gene can be attenuated by introducing a mutation into the gene on the chromosome so that the intracellular activity of the protein encoded by the gene is decreased as compared with an unmodified strain. Mutations which result in attenuation of expression of the gene include the replacement of one base or more to cause an amino acid substitution in the protein encoded by the gene (missense mutation), introduction of a stop codon (nonsense mutation), deletion or insertion of one or two bases to cause a frame shift, insertion of a drug-resistance gene, or deletion of a part of the gene or the entire gene (Qiu, Z. and Goodman, M.F., J. Biol. Chem., 272, 8611-8617 (1997); Kwon, D. H. et al, J. Antimicrob. Chemother., 46, 793-796 (2000)). Expression of the artI gene can also be attenuated by modifying an expression regulating sequence such as the promoter, the Shine-Dalgarno (SD) sequence, etc. (WO95/34672, Carrier, T.A. and Keasling, J.D., Biotechnol Prog 15, 58-64 (1999)).

[0040] For example, the following methods may be employed to introduce a mutation by gene recombination. A mutant gene encoding a mutant protein with decreased activity is prepared, and the bacterium to be modified is transformed with a DNA fragment containing the mutant gene. Then, the native gene on the chromosome is replaced with the mutant gene by homologous recombination, and the resulting strain is selected. Gene replacement using homologous recombination can be conducted by employing a linear DNA, which is known as "Red-driven integration" (Datsenko, K.A. and Wanner, B.L., Proc. Natl. Acad. Sci. USA, 97, 12, p 6640-6645 (2000)), or by employing a plasmid containing a temperature-sensitive replication origin (U.S. Patent 6,303,383 or JP 05-007491A). Furthermore, site-specific mutation by gene substitution can also be incorporated using homologous recombination such as set forth above using a plasmid which is unable to replicate in the host.

[0041] Expression of the gene can also be attenuated by inserting a transposon or an IS factor into the coding region of the gene (U.S. Patent No. 5,175,107), or by conventional methods, such as by mutagenesis with UV irradiation or nitrosoguanidine (N-methyl-N'-nitro-N-nitrosoguanidine).

[0042] The gene can also be inactivated by conventional methods, such as by mutagenesis using UV irradiation or nitrosoguanidine (N-methyl-N'-nitro-N-nitrosoguanidine), site-directed mutagenesis, gene disruption using homologous recombination, or/and insertion-deletion mutagenesis (Yu, D. et al., Proc. Natl. Acad. Sci. USA, 2000, 97:12: 5978-83 and Datsenko, K.A. and Wanner, B.L., Proc. Natl. Acad. Sci. USA, 2000, 97:12: 6640-45), also called "Red-driven integration".

[0043] Methods for preparation of plasmid DNA, digestion and ligation of DNA, transformation, selection of oligonucleotides as primers, and the like may be ordinary methods well known to one skilled in the art. These methods are described, for instance, in Sambrook, J., Fritsch, E.F., and Maniatis, T., "Molecular Cloning A Laboratory Manual, Second Edition", Cold Spring Harbor Laboratory Press (1989).

L-arginine producing bacteria

[0044] Bacteria which are modified to attenuate expression of one or several genes encoding an L-arginine transporter and which are able to produce L-arginine may be employed.

[0045] The bacterium can be obtained by inactivating one or several genes encoding an L-arginine transporter in a bacterium which has a native or inherent ability to produce L-arginine. Alternatively, the bacterium can be obtained by imparting the ability to produce L-arginine to a bacterium already having an inactivated one or several genes encoding an L-arginine transporter.

[0046] Examples of parent strains which can be used to derive L-arginine-producing bacteria include, but are not limited to, strains belonging to the genus Escherichia, such as E. coli strain 237 (VKPM B-7925) (U.S. Patent Application 2002/058315 Al) and its derivative strains harboring mutant N-acetylglutamate synthase (Russian Patent Application No. 2001112869), E. coli strain 382 (VKPM B-7926) (EP1170358A1), an arginine-producing strain into which the argA gene encoding N-acetylglutamate synthetase has been introduced (EP1170361A1), and the like.

[0047] Examples of parent strains which can be used to derive L-arginine producing bacteria also include strains in which expression of one or more genes encoding L-arginine biosynthetic enzymes is/are enhanced. Examples of such genes include the genes encoding N-acetylglutamyl phosphate reductase (argC), ornithine acetyl transferase (argJ), N-acetylglutamate kinase (argB), acetylomithine transaminase (argD), ornithine carbamoyl transferase (argF), argininosuccinic acid synthetase (argG), argininosuccinic acid lyase (argH), and carbamoyl phosphate synthetase (carAB). The abbreviations in parentheses after the enzyme names represent the gene names.

2. Method of the present invention

[0048] Exemplary methods include producing L-arginine by cultivating the bacterium as described herein in a culture medium to produce and secrete L-arginine into the medium, and collecting L-arginine from the medium.

[0049] The cultivation, collection, and purification of L-arginine from the medium and the like may be performed by conventional fermentation methods wherein an amino acid is produced using a bacterium.

[0050] The medium used for the culture may be either a synthetic or natural medium, so long as the medium includes a carbon source and a nitrogen source and minerals and, if necessary, appropriate amounts of nutrients which the bacterium requires for growth. The carbon source may include various carbohydrates such as glucose and sucrose, and various organic acids. Depending on the mode of assimilation of the chosen microorganism, alcohol, including ethanol and glycerol, may be used. As the nitrogen source, various ammonium salts such as ammonia and ammonium sulfate, other nitrogen compounds such as amines, a natural nitrogen source such as peptone, soybean-hydrolysate, and digested fermentative microorganism can be used. As minerals, potassium monophosphate, magnesium sulfate, sodium chloride, ferrous sulfate, manganese sulfate, calcium chloride, and the like can be used. As vitamins, thiamine, yeast extract, and the like, can be used.

[0051] The cultivation is preferably performed under aerobic conditions, such as a shaking culture, and a stirring culture with aeration, at a temperature of 20 to 40 °C, preferably 30 to 38 °C. The pH of the culture is usually between 5 and 9, preferably between 6.5 and 7.2. The pH of the culture can be adjusted with ammonia, calcium carbonate, various acids, various bases, and buffers. Usually, 1 to 5-day cultivation leads to accumulation of L-arginine in the liquid medium.

[0052] After cultivation, solids such as cells can be removed from the liquid medium by centrifugation or membrane filtration, and then L-arginine can be collected and purified by ion-exchange, concentration, and/or crystallization methods.

Examples

[0053] The present invention will be more concretely explained below with reference to the following non-limiting Examples.

Example 1. Construction of a strain with an inactivated artI gene.

1. Deletion of the artI gene.

[0054] The artI gene in a bacterial strain was deleted by the method initially developed by Datsenko, K.A. and Wanner, B.L. (Proc. Natl. Acad. Sci. USA, 2000, 97(12), 6640-6645) called "Red-driven integration". According to this procedure, the PCR primers P1(SEQ ID NO: 11) and P2 (SEQ ID NO: 12), which are homologous to both the region adjacent to the artI gene and the gene which confers antibiotic resistance in the template plasmid, were constructed. The plasmid pMW118-attL-Cm-attR (WO 05/010175) was used as the template in the PCR reaction. Conditions for PCR were as follows: initial DNA denaturation for 5 min at 95 °C, followed by 25 cycles of denaturation at 95 °C for 30 sec, annealing at 54 °C for 30 sec, elongation at 72 °C for 40 sec; and the final elongation for 5 min at +72 °C.

[0055] The 1.7 kb PCR product (Fig. 1) was purified from an agarose gel and used for electroporation of the E. coli strain MG1655 (ATCC 700926), which contains the plasmid pKD46. The pKD46 plasmid (Datsenko, K.A. and Wanner, B.L., Proc. Natl. Acad. Sci. USA, 2000, 97:12:6640-45) contains a temperature-sensitive replication origin, and includes a 2,154 nucleotide DNA fragment of phage λ (nucleotide positions 31088 to 33241, GenBank accession no. J02459), as well as the genes of the λ Red homologous recombination system (y, β, exo genes) which are under the control of the arabinose-inducible P_araB promoter. The pKD46 plasmid is necessary for integration of the PCR product into the chromosome of the MG1655 strain. The strain MG1655 can be obtained from American Type Culture Collection. (P.O. Box 1549 Manassas, VA 20108, U.S.A.).

[0056] Electrocompetent cells were prepared as follows: E. coli MG1655/pKD46 cells were grown overnight at 30 °C in LB medium containing ampicillin (100 mg/l), and the culture was diluted 100 times with 5 ml of SOB medium (Sambrook et al, "Molecular Cloning: A Laboratory Manual, Second Edition", Cold Spring Harbor Laboratory Press, 1989) containing ampicillin and L-arabinose (1 mM). The cells were grown with aeration at 30 °C to an OD₆₀₀ of ≈0.6 and then were made electrocompetent by concentrating 100-fold and washing three times with ice-cold deionized H₂O. Electroporation was performed using 70 µl of cells and ≈100 ng of the PCR product. Cells after electroporation were incubated with 1 ml of SOC medium (Sambrook et al, "Molecular Cloning: A Laboratory Manual, Second Edition", Cold Spring Harbor Laboratory Press, 1989) at 37 °C for 2.5 hours and then were plated onto L-agar containing chloramphenicol (30 µg/ml) and grown at 37 °C to select Cm^R recombinants. Then, to eliminate the pKD46 plasmid, two passages on L-agar with Cm at 42 °C were performed and the colonies were tested for sensitivity to ampicillin.

2. Verification of the artI gene deletion by PCR.

[0057] The mutants in which the artI gene has been deleted, and which were marked with Cm resistance gene, were verified by PCR using the locus-specific primers P3 (SEQ ID NO: 13) and P4 (SEQ ID NO: 14). For this purpose, a freshly isolated colony was suspended in 20µl water and then 1 µl of the suspension was used for PCR. The temperature profile follows: initial DNA denaturation for 5 min at 95 °C; then 30 cycles of denaturation at 95 °C for 30 sec, annealing at 55 °C for 30 sec and elongation at 72 °C for 1 min; the final elongation for 5 min at 72 °C. The PCR product obtained in the PCR reaction using the cells of the parental artI⁺ strain MG1655 as the template was 803 bp in length. The PCR product obtained in the PCR reaction using the cells of the mutant MG1655 ΔartI::cat strain as the template was 1453 nucleotides in length (Fig.2). The mutant strain was named MG1655ΔartI.

Examples 2. Construction of a strain with an inactivated artPIQM-artJ cluster.

1. Deletion of the artPIQM-artJ cluster.

[0058] The artPIQM-artJ cluster in a bacterial strain was deleted by Red-driven integration. According to this procedure, the PCR primers P5(SEQ ID NO: 15) and P6 (SEQ ID NO: 16), which are homologous to both the region adjacent to the artPIQM-artJ cluster and the gene which confers antibiotic resistance in the template plasmid, were constructed. The plasmid pMW118-attL-Cm-attR (WO 05/010175) was used as the template in the PCR reaction. Conditions for PCR were as described above.

[0059] The 1.7 kb PCR product (Fig. 1) was purified from an agarose gel and used for electroporation of the E. coli strain MG1655 (ATCC 700926), which contains the plasmid pKD46

[0060] Electrocompetent cells were prepared as as described above. Electroporation was performed using 70 µl of cells and ≈ 100 ng of the PCR product. Cells after electroporation were incubated with 1 ml of SOC medium at 37 °C for 2.5 hours and then were plated onto L-agar containing chloramphenicol (30 µg/ml) and grown at 37 °C to select Cm^R recombinants. Then, to eliminate the pKD46 plasmid, two passages on L-agar with Cm at 42 °C were performed and the colonies were tested for sensitivity to ampicillin.

2. Verification of deletion of the artPIOM-artJ cluster by PCR.

[0061] The mutants in which the artPIQM-artJ cluster has been deleted, and which were marked with Cm resistance gene, were verified by PCR using the locus-specific primers P7 (SEQ ID NO: 17) and P8 (SEQ ID NO: 18) as described above. The PCR product obtained in the PCR reaction using the cells of the mutant MG1655 ΔartPIQM-artJ::cat strain as the template was 1,5 kb nucleotides in length (Fig.2). ). The mutant strain was named MG1655ΔartPIMQ-artJ.

Example 3. Production of L-arginine by E. coli 382ΔartI and E. coli 382ΔartPIMQ-artJ.

[0062] To test the effect of inactivation of the artI gene or artPIMQ-artJ cluster on L-arginine production, DNA fragments from the chromosome of the above-described E. coli MG1655ΔartI and E. coli MG1655ΔartPIMQ-artJ were transferred to the L-arginine producing E. coli strain 382 by P1 transduction (Miller, J.H. (1972) Experiments in Molecular Genetics, Cold Spring Harbor Lab. Press, Plainview, NY) to obtain E. coli 382ΔartI and 382ΔartPIMQ-artJ strains, respectively. The strain 382 has been deposited in the Russian National Collection of Industrial Microorganisms (VKPM) (Russia, 117545 Moscow, 1 Dorozhny proezd, 1) on April 10, 2000 under accession number VKPM B-7926 and then converted to a deposit under the Budapest Treaty on May 18, 2001.

[0063] The E. coli strains, 382, 382ΔartI and 382ΔartPIMQ-artJ, were separately cultivated with shaking at 37 °C for 18 hours in 3 ml of nutrient broth, and 0.3 ml of the cultures were inoculated into 2 ml of a fermentation medium in 20 x 200-mm test tubes and cultivated at 32 °C for 48 hours on a rotary shaker.

[0064] After the cultivation, the amount of L-arginine which has accumulated in the medium was determined by paper chromatography using the following mobile phase: butanol : acetic acid : water = 4 : 1 : 1 (v/v). A solution of ninhydrin (2%) in acetone was used as a visualizing reagent. A spot containing L-arginine was cut out, the L-arginine was eluted with 0.5% water solution of CdCl₂, and the amount of L-arginine was estimated spectrophotometrically at 540 nm.

[0065] The composition of the fermentation medium (g/l) is as follows:

Glucose	48.0
(NH4)2SO4	35.0
KH2PO4	2.0
MgSO4·7H2O	1.0
Thiamine HCl	0.0002
Yeast extract	1.0
L-isoleucine	0.1
CaCO3	5.0

[0066] Glucose and magnesium sulfate are sterilized separately. CaCO₃ is dry-heat sterilized at 180 C for 2 hours. The pH is adjusted to 7.0.

[0067] The results of test tube fermentations are shown in Table 1. As it can be seen from the Table 1, strains with inactivated artI gene or artPIMQ-artJ cluster caused a higher amount of accumulation of L-arginine as compared with parent L-arginine producing E. coli strain 382.

Table 1

Strain	Amount of L-arginine, g/l
382	12.0 ± 0.1
382ΔartI	14.3 ± 0.1
382ΔartPIMQ-artJ	13.4 ± 0.1

[0068] While the invention has been described in detail with reference to preferred embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. All the cited references herein are incorporated as a part of this application by reference.

Claims

1. An L-arginine producing bacterium of the Enterobacteriaceae family, wherein said bacterium has been modified to attenuate expression of one or several genes encoding an L-arginine transporter.

2. The bacterium according to claim 1, wherein said bacterium has been modified to attenuate expression of artI gene.

3. The bacterium according to claim 2, wherein expression of the artI gene is attenuated by inactivation of the artI gene.

4. The bacterium according to claim 1, wherein said bacterium has been modified to attenuate expression of artPIQM-artJ cluster.

5. The bacterium according to claim 4, wherein expression of the artPIQM-artJ cluster is attenuated by inactivation of the artPIQM-artJ cluster

6. The bacterium according to claim 1, wherein said bacterium belongs to the genus Escherichia.

7. The bacterium according to claim 1, wherein said bacterium belongs to the genus Pantoea.

8. The bacterium according to claim 6, wherein said bacterium is Escherichia coli.

9. A method for producing L-arginine comprising:

- cultivating the bacterium according to any of claims 1 to 8 in a medium, and

- collecting L-arginine from the medium.

Drawing